Digital data processing systems, or computer systems, allow users to organize and maintain large amounts of information from a number of applications programs, and to generate information for use by those or other applications programs. In many cases, it is desirable to divide the massive amounts of information that a user may have available to him on the computer into a number of diverse items. To organize the information, the information may be classified according to a multiple-level classification scheme. Familiar examples of such classification schemes include the Dewey Decimal System for classifying books in a library, division of a reference book on a particular topic into a number of diverse parts, each perhaps having a plurality of subsidiary subdivisions, and so forth, and organization of documents into filing cabinets, drawers and folders, all of which provide a tree-like classification structure such as would be provided in, for example, an outline.
Several examples may help explain the use of hierarchical organization of information in a computer system. Information used in, for example, an accounting program may be divided into, at a first level, accounts receivable information, accounts payable information, asset information, liability information, and so forth. The accounts receivable information, in turn, may be organized by the party owing the receivable, and then further divided by the aging of the receivable. Similarly, the accounts payable information may be organized by the party to whom the payable is owed and then further divided by the aging of the payable. The assets may be divided into the type of asset, that is, whether the asset is current or fixed. Continuing with the example, the current assets may be further divided by whether they are cash or cash equivalents, with cash being further organized by the banks in which it is deposited and cash equivalents being further organized into the type of asset, such as stock, bond, and so forth. Similarly, the fixed assets may be organized by whether the asset is depreciable or not depreciable, further organized by location, and so forth.
Similarly, using a mail applications program on a computer system, a user may generate memoranda and transmit them to other users on the computer system. In addition, using the mail applications program, the user may receive memoranda generated by other users. In many cases, a user may wish to organize the received memoranda by originator, dividing the memoranda according to the originators' departments, sections, or other subdivisions in which the company may be partitioned. If the memoranda are organized in this manner, the user, by specifying a subdivision may easily retrieve only the memoranda from originators in the specified subdivision, which may reduce the time required to review his mail if he is not interested in memoranda from other subdivisions. Alternatively, or additionally, a user may wish to organize the memoranda by subject matter, which may also be divided into a plurality of levels, and review memoranda only relating to particular subjects.